In some instances in the clinical analysis of blood samples, it is desirable that the patient sample be diluted so as to retest an out-of-range condition. By adding the sample to a diluent, or vice versa, in a predetermined ratio, e.g., 1:1, the out-of-range value is reduced to a within-range reading. For example, the condition can occur in assays for glucose.
Prior art approaches have been to eject the sample or the diluent into an empty container until the desired volume is achieved, and then the other liquid is added. In most, if not all of such approaches, no care is given to the location of the bottom of the container. The dispenser tip should not be unduly submerged during liquid ejection, lest substantial amounts of the liquid end up on the surface of the withdrawn tip instead of inside the container. Hence, most such approaches err on the side of spacing the tip so high above the bottom surface of the container that the liquid is ejected in drops, rather than a continuous stream. That is, the distance is too great to allow a continuous stream to flow.
Ejection as drops produces no problem, until the last amount of liquid is ejected. That last amount tends to hang as a pendant drop, with or without perfusion up the outside surface of the dispensing tip. We have discovered that as much as 25% of the desired volume can end up in such a pendant drop, rather than in the liquid in the container, so that the dilution ratio inside the container can be severely altered. Such a dilution method is unacceptable.
This invention is not the first to appreciate the importance in knowing where the bottom of the receiving container actually is, in each instance. European Patent Publication 223,758 teaches the sensing of the bottom of each container, simply by measuring the increase in axial force on the pipette that occurs when it strikes the bottom. However, this is subject to error. For example, if the pipette strikes a side wall or side wall projection of the container before reaching bottom, an axial component of force can still be delivered. This is particularly a problem for non-cylindrical containers or those with sloping side walls. The result in such case can be a false reading of bottoming. Thus, the technique used by this European application places an undue premium on proper location of the dispenser tip in the X-Y plane, vis-a-vis the container, to avoid side wall contact. Otherwise, the technique of that application will not in fact always determine the actual bottom. Still further, the axial force on the pipette is measured by this technique using a sensor added only for this purpose. A better technique would use sensors already in place.